1. Field of the Invention
The present invention relates to a cooling device for a vehicle motor used in an electric automobile.
2. Description of the Prior Art
In recent years, in view of environmental problems, electric automobiles have been extensively developed. Such a type using wheel motors has been known that each motor is directly connected to a corresponding wheel through a reduction gear.
In such arrangement that each wheel is directly connected to the corresponding motor through the reduction gear, it is necessary to prevent interference of a vehicle body with the motor and the reduction gear during vertical movements and steering operations of the wheels. This forms a restriction with respect to an installation space. Therefore, it is required to minimize the sizes of the motor and the reduction gear for disposing them in a limited space. Meanwhile, the electric automobiles are required to use motors having small and light structures as well as large output torques so as to obtain high powers relative to the vehicle weights.
However, in order to reduce the sizes and weight of the motor and to increase the output torque thereof, it is necessary to flow a large current through a coil of the motor, so that the coil generates a large amount of heat, which may cause burning of the coil. This is due to the facts that thermal resistance of the coil is large, and the heat resistance of the iron core which serves to transfer the heat to a casing of the motor is also high because the iron core has stacked layers of silicon steel plates, so that, as heating density of the coil increases, the heat accumulated in the coil increases, resulting in a rapid increase of the temperature of the coil. Therefore, it is important to suppress the heating of the coil in the overloaded state for cooling it in order to obtain an intended performance of the motor.
Such a manner for cooling has already been known that a liquid coolant such as oil is held in the motor to directly immerse the coil end in the coolant so that a stirring operation by a rotor may serve to cool the coil.
Since the conventional cooling device, however, is designed to cool the coil only by the stirring of the coolant by rotor, the coil end may not be cooled uniformly, which causes a local heating of the coil and thus reduction of durability of an insulator for the coil, resulting in the burning of the motor. Further, if the amount of the oil is increased so as to fully cool the coil end, the output power of the motor will be reduced due to the stirring resistance against the rotor, and thus a large power loss is inevitable for obtaining an intended power.
Further, due to the restriction on the installation space in the electric automobile described before, it is extremely difficult to find the installation space for the cooling device. Even if the cooling device were disposed under such conditions, the cooling device would interfere with the wheel during a vertical movement and/or steering operation of the wheel.
Accordingly, it is an object of the invention to provide a cooling device for a vehicle motor, overcoming the above noted problems, by which an electric motor can have small size and a high output power.